Recently, electric vehicles, hybrid vehicles and fuel cell vehicles that mount an electric storage and mount a motor as a driving device have been attracting attention as environmentally friendly vehicles. In such vehicles, regenerative braking, in which kinetic energy is converted by a motor to electric energy and returned to a battery when the brake is applied to the vehicle, is positively adopted to improve energy efficiency.
Among such vehicles, a hybrid vehicle is provided with a generator driven by an engine, in order to make up for the shortcomings of an electric vehicle that a large capacity battery is necessary to attain long running distance so that the vehicle becomes heavy and charging takes long time.
In regenerative braking, however, it is possible that the regenerated electric power and the electric power generated by the generator are both applied across the battery terminals and, dependent on the state of charge of the battery, the voltage across the terminals could increase past the tolerable voltage. If an excessive voltage should be applied across the terminals, electrolyte in the battery might possibly decompose, or gas or heat could be generated, making shorter the battery life.
Japanese Patent Laying-Open No. 8-79911 discloses a hybrid-type electric vehicle solving such a problem, in which application of excessive voltage across the terminal at the time of regenerative braking can be prevented.
Battery life much depends on battery temperature. The heat generated in the battery is given by I2R, where I represents battery current and R represents internal resistance. If an excessive current flows even for a short period of time, the heat builds up in proportion to the square of the current. Therefore, not only from the viewpoint of state of charge but also from the viewpoint of battery temperature, it is necessary to regulate the electric power derived from regenerative braking and the electric power generated by the generator not to be applied at one time across the battery terminals. Specifically, consideration is necessary even if the state of charge is low, to prevent instantaneous peak of charging current from attaining excessively high.
On the other hand, it is not the case that the lower the battery temperature, the better. There is an optimal temperature range for use. Therefore, the hybrid vehicle disclosed in Japanese Patent Laying-Open No. 8-79911 still has a room for improvement.
An object of the present invention is to provide a vehicle drive device attaining improved energy efficiency while protecting the electric storage.